Method and device for inspecting and/or controlling thermally produced mechanical joints

ABSTRACT

The invention is concerned with a method and apparatus for effecting a thermal joint between two materials while sensing the condition of the joint during the joining process. The described example utilizes an electron beam as the heating means, there being sensing elements located in the region of the joint being made. The elements are sensitive to temperature or sonic or electric signals, and their outputs are processed to show the state of the joint, or to control the means making the joint. The sensing elements may be fixed to the workpieces or move with the electron beam along the path of the joint.

219-121 SR ,7 ltiTRC l- DR 39648s009 United States Patent 1151 3,648,009

Steigerwald 1 Mar. 7, 1972 METHOD AND DEVICE FOR 2,371,636 3/ 1945 McConnell ..324/62 INSPECTING AND/OR CONTROLLING 2,616,014 10/1952 Ellerby .1 ....2l9/109 THERMAL PRODUCE 3%33121 3/1323 l3 "'a""' "@1313? orman o MECHANICAL JOINTS 3,410,983 11/1968 Deutsch et al.. ....219/l09 [72] Inventor: Karl Heinz Steigerwald, 55 Haderunstr. 3,418,548 12/1968 Raphael ..318/ 18 1a, Munich, Germany Primary Examiner-J. V. Truhe [22] Filed May 1970 Assistant Examiner-Gale R. Peterson [21] Appl. No.: 34,775 Attorney-Sandoe,Neill,Schottler& Wikstrom [30] Foreign Application Priority Data [57] ABSTRACT The invention is concerned with a method and apparatus for May 6, 1969 Germany ..P 19 23 132.4 effecting a thermal joint between two materials while sensing the condition of the joint during the joining process. The "219/121 EMzlg/ 121 described example utilizes an electron beam as the heating meansthere being Sensing elementslocatedin the region of [58] of 4 the joint being made. The elements are sensitive to temperature or sonic or electric signals, and their outputs are processed to show the state of the joint, or to control the [56] Reieremes Cited means making the joint. The sensing elements may be fixed to UNITED STATES TEN S tlz etgvorltpiteces or move with the electron beam along the path 0 e om. 1,948,337 2/1934 Crawford ..219/l09 J 2,089,015 8/1937 Bucknam et al ..2l9/ 135 43 Claims, 27 Drawing Figures meme March 1, 1972 3,648,009

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Patented March 7, 1972 3,648,009

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KN X 4 may METHOD AND DEVICE FOR INSPECTING AND/OR CONTROLLING THERMALLY PRODUCED MECHANICAL JOINTS PRIOR APPLICATION In Federal Republic of Germany on May 6th, 1969 and numbered P 19 23 132.4

The invention relates to a method and a device for controlling and/or inspecting mechanical joints produced by a thermal jointing process, more particularly welding.

When joining two workpieces by thermal joining processes such as welding or brazing, it is sometimes necessary to ascertain with reliability the quality of the joint. Hitherto two kinds of methods have been used for this: first, so-called destructive tests, in which the joint is destroyed by mechanical stressing, or separated in any other method, and the properties thereof investigated, and secondly, investigating methods without destruction such as X-ray exposures or ultrasonic sounding.

Opinion is to the effect that known nondestructive tests of weld seams do not really permit a fundamentally reliable statement of their quality. For this reason, thermal joining methods in highly stressed joints are not used, or used only with reluctance, and structural configurations and saving of costs are not considered.

More particularly, difficulties are encountered in'high-performance welding processes such as resistance welding, electron beam or laser beam welding and friction welding, in which heating of the workpieces to be connected can be concentrated at the region of the surfaces to be connected, so that both the necessary quantity of heat and the duration of heat action can be reduced to a minimum. While, weld technological advantages can be obtained, processing technological difficulties may arise. These consist particularly in that it is essential for the heat supplied to exactly engage the workpiece region which has to be heated to obtain the joint, and that the heating operation itself takes the correct welding path. If this is not the case, then the heat applied does not sufiice to produce a joint of the required extent, or the joining process is considerably disturbed and joining faults occur.

The object of the present invention is to provide a method and a device which enable the quality of a joint, especially a weld seam, formed in accordance with the principle of intermelting materials of two or more workpieces along their contact surfaces, to have an increased reliability without material destruction than has hitherto been possible.

According to the invention, during a thermal joining process, conditional changes in the materials joined occur in the region of the joint being produced, which are used to gauge the quality of the joint.

In a method in accordance with the invention, the required data concerning the quality of the joint are acquired during the joining process, so that normally there is no longer any need to carry out a subsequent test. The method of the invention is particularly reliable because minor deviations from ideal conditions are reflected in relatively large changes in the timed and local distribution of quality values, such as the temperature distribution occurring during the jointing process.

The method of the invention results in particularly clear indications of the jointing obtained in such thermal processes, the course of which is such that during the thermal production of the joint between two or more workpieces each point of the workpieces involved passes through only one definite cycle of conditional changes, such as, a temperature cycle, from a starting temperature up to a temperature maximum and back to the starting temperature. This occurs for example in resistance welding, friction welding and beam welding such as that employing an electron laser beam; disturbances occurring in other processes are avoided and the welding process carried out accurately.

In most cases it suffices if the conditional changes and distribution of one state value is ascertained, the values of state existing being measured at several predetermined points of the point of the workpieces to be joined a variation with time of the value of the said state will occur which is characteristic of the action of the joining process, and which assumes characteristic relation to, the changes of this value state at other points, so that with an overall observation of the value states at several measuring-points a very reliable estimate concerning the course of the jointing process and the quality of the joint produced thereby, is possible. When using the invention to produce a joint along a seam by moving a thermal tool such as an electron beam, relative to the workpieces along the seam, it is preferred to observe the distribution of the said state value at measuring points located on both sides of the seam. In such a method of operation, asymmetry of the energy feed is quite noticeable.

The measuring points may be progressively moved with the tool, for example, continuously; this simplifies the measurement process. The measuring points may also be fixed relative to the workpieces; this is advantageous with regard to accuracy of measurement. Progressive movement of the measuring points may also be obtained by tapping measuring signals successively and progressively from several stationary measuring points. i

Since the workpieces to be connected are often accessible only with difficulty during joining, for example, in joining processes which are carried out in a vacuum, a method of operation is obviously advantageous in which an image of the region of the joint to be produced and the distribution of the state of the joint is produced from the distribution of a value of state in the'workpieces, and an image thereof produced in form of an intensity distribution. A further feature of this process is that a conversion of the measured value of state is effected in to visible light, and the distribution of the value of state is shown in the form of the brightness distribution of the image.

The process of the inventionis particularly suitable for repeated production of identical joints in identical workpieces. Thus, the process may proceed by way of a number of tests using working conditions varied in a predetermined manner, measurements of the joint state being recorded in accordance with a timed distribution program, followed by destructive investigation of the joints obtained, to ascertain a best jointing procedure; then the working conditions corresponding to this best procedure are used to produce all subsequent joints, any serious deviation found by measurement in a joint being sufficient to cause rejection thereof. The accuracy of the process increases with the number of test points.

A further advantage of the process in accordance with the invention is that a measurement of a single parameter such as temperature may be derived and used to control or automati cally regulate the thermal jointing process. When producing a joint by means of a thermal tool moved along a seam it is preferred to proceed in such a manner that the values of the parameter measured are derived from measuring points which are located in front of the tool in its direction of feed. The working speed of the joining process must of course be taken into account.

In one embodiment of the process in accordance with the invention the temperature distribution in the workpieces is used as the parameter-serving to indicate changes of state in the joint. This value is particularly characteristic of thermal joining processes, because minor deviations from ideal conditions show up as large changes in the timed and local temperature distribution characteristics. This embodiment of the process may be applied to nondestructive material testing of workpieces, whereby a heater current is fed into a region of the workpiece to be investigated and the temperature distribu tion resulting therefrom is plotted and used as measure of the quality of the workpiece region investigated.

The process of the invention may also be carried out by measuring parameters. Thus during a joining process a workpiece is supplid with input signals of a kind which propagates through the workpiece material, and transmission measureworkpieces to be joined. During the joining process, at each ments are madethroughout the joining region. Thus pressure signals may be used and a pressure signal measured at a point spatially separated from the input point thereof. Preferably continuous or impulsed sound signals, more particularly ultrasonic signals, are used as pressure signals. The transit time of the signals is measured between the input and measuring points; it is possible to use reflection from or transmission of the signals through the region of the joint produced. In a particularly sensitive embodiment sound signals are synchronously supplied at two input points and the resultant interference pattern evaluated.

In an alternative simple and sensitive method, magnetic signals are used as input signals and a magnetic field measured at least at one measuring point. An alternating magnetic field may be used as input signal, and the resultant flux density measured at a predetermined point. A particularly high sensitivity is obtained if the alternating flux and the measuring point thereof are located close to the region of the joint to be produced. In electrically conductive metal workpieces the use of an alternating magnetic field causes circulating currents which in turn cause a magnetic field which can be detected at a distance. The distribution of the circulating currents and the magnetic flux caused thereby vary considerably as soon as a rigid joint has been formed between two workpieces to be connected.

In the embodiments of the invention which use a separate input signal when producing a thermal joint between two workpieces along a scam by moving a thermal tool such as an electron beam, relative to the workpieces along the seam, it is preferable for the input point to be progressively moved together with the tool.

A device for carrying out a process in accordance with the invention is such that to evaluate the condition of the joint, a plurality of measuring elements are provided in a predetermined arrangement in or on the workpieces to be joined. When producing a wholly or partially continuous joint between two parts of a workpiece, such measuring elements are preferably provided on both sides of the workpiece. When producing a connection along a seam it is preferable to provide measuring elements on both sides of the seam. A symmetrical arrangement of the measuring elements provides an accurate evaluation of the quality of the joint from the differences between signals existing between equivalent symmetrically placed measuring elements.

The measuring elements may be either stationary or movable relative to the workpiece. In the former case, the measuring elements may be fixed to the workpiece, or if repetitional production is envisaged, the measuring elements are mounted on a holder so as to be detachably connected with the workpieces to be joined, so that when the holder is applied to the workpieces, the measuring elements are applied at predetermined measuring points on the workpieces; they may be resiliently supported against the workpiece surface or located at a slight clearance from the workpiece surface, as when the measuring elements are radiation sensitive.

When the measuring elements are moved relative to the workpiece it is preferable that they are mounted on a holder. In this case the holder together with the thermal tool producing the joint are movable relative to the workpieces, and during such movement the measuring elements move along predetermined movement paths. The measuring elements may be held against the workpieces by resilient sliding means. It is, however, possible for the measuring elements to be held at a slight clearance from the workpiece surface.

A particularly useful embodiment is one in which a program generator which is synchronized with the progress of the jointing operation, supplies a program of optimum values of the measuring signals generated by the measuring elements during joining. There is also a comparison device which compares the optimum values with the measured signals and supplies cor- -responding difference signals. In such an embodiment it is possible to obtain an estimate of the degree of deviation from a required maximum joint quality from the differences between the optimum value signals and the signals supplied by the measuring elements; this estimate may be used to reject bad joints.

A further embodiment includes a regulating device actuated by the signals of the measuring elements, acting on controls of the joining process so that the measuring signals or signals derived therefrom remain within a predetermined range of values.

Instead of observing the distribution of the state of value directly on the workpiece, which sometimes creates difficulties as for example, in a joining process operating in a vacuum, it is also possible to choose an analogue image of workpieces", thus an image system responding to temperature differences, more especially an optical image system responding to heat radiation, is provided for the region of the workpiece to be joined, the measuring elements being located in the image area of the image system instead of in or on the workpieces.

The nature of the measuring elements will depend on the value to be measured. If the temperature is to be regarded as the criterion, the measuring elements may be formed as resistance thermometers, thermoelements, radiation gauges or expansion thermometers. A less obvious sensor which is applicable in jointing processes operating at high temperatures in ferromagnetic materials, consists in that measuring elements are used which respond to the magnetic permeability of the workpieces. Such measuring elements may be used particularly well to feel" the isotherms corresponding to the path of the Curie temperature.

When a separately fed input signal is used, the apparatus in accordance with the invention is preferably so formed that a signal generator is coupled, at least at one predetermined input point of a workpiece to be joined, the generator supplying a signal to be processed by the measuring elements used after traversing the workpiece material. The signal generator together with the thermal tool used for producing the joint may be movable relative to the workpiece. When producing a thermal joint between two workpieces along a seam, it is expedient for the signal generator to be coupled to one workpiece and at least one measuring element to the other workpiece, so that the proportion of the signal received by the measuring element depends upon the state of the seam. As already mentioned, a preferred form uses an alternating pressure generator, such as a supersonic generator. If it is intended to use the particularly sensitive interference method already mentioned, two synchronously operating signal generators with predetermined spacing may be located parallel to the direction of the seam; the other workpiece then also has two spaced measuring elements provided thereon. The circulating current method may be used when producing joints between electrically conductive workpieces the apparatus comprising a signal generator producing an alternating magnetic field and at least one measuring element responding to alternating magnetic flux, both items being so located that they act in the region of the thermal tool producing the joint.

Embodiments of the invention are described below in detail by way of the drawings, in which:

FIG. 1 is a schematic plan view of two workpieces during their jointing along a seam by means of electron beam welding,

FIG. 2 is a schematic view of the underside of the workpieces shown in FIG. 1,

FIG. 3 is a section taken on the line A,,B, of FIG. 1,

FIG. 4 is a graph of the time-temperature distribution at the measuring points indicated in FIG. 1,

FIG. 5 is a graph of the temperature distribution on both sides of a weld seam of the kind shown in FIGS. 1 to 3,

FIGS. 6, 7 and 8 are views similar to FIGS. 1, 2 and 3, but with the electron beam diverging from the normal out of the seam surface,

FIGS. 9, 10 and 11 are views similar to FIGS. 1, 2 and 3, but in which the electron beam is incorrectly focused or moved,

FIGS. l2, l3 and 14 are views corresponding to FIGS. 9, l0 and 11, but in which the curvature of the electron beam path is restricted.

FIGS. to 18 are views of various arrangements of measuring elements, i

FIG. 19 is a view of a holder for the measuring elements, adapted to be detachably connected to the workpieces,

FIG. 20 is a view of a holder for the measuring elements adapted to be displaceable together with a thermal tool, relative to the workpieces,

FIG. 21 is a partially perspective view of a possible embodiment of the invention,

FIG. 22 is a view of a further possible embodiment of the invention,

FIG. 23 is a similar view to that of FIG. 1 showing a different embodiment of the process of the invention,

FIG. 24 is a similar view to that of FIG. 2, but showing a further embodiment of the process in accordance with the invention,

FIG. 25 is a plan view of two workpieces during their joining along aseam by welding, to illustrate an alternative embodiment of the invention,

FIGS. 26 and 27 are graphs of the magnetic alternating flux density near the weld seam of the process shown in FIG. 25.

Most of the foregoing figures show embodiments in which the tern; :rature of the workpieces is measured as a weld criterion; it is, however, to be understood that the process or method described and shown may be carried out by referring to other properties of the material being welded.

FIG. 1 shows two workpieces 2 and 4 which are to be butt joined at their contact surfaces 6 by electron beam welding. The welding electron beam 8 impinges the material surface at right angles to the plane of the drawing and moves relative to the workpieces in the direction of arrow 26. It leaves behind a joint 28. The line A,,B,, indicates a cross-sectional area moved along with the welding electron beam 8, and further cross-sectional areas A -B A -B A,,-B,, are envisaged which follow at predetermined distances behind the welding surface A,,B,,.,. The lines 10, 12 and 14 indicate possible isotherms which characterize the temperature distribution present at a certain time in the three observation areas. FIG. 4 is a graph showing the temperature T at the measuring points l6, 18, 20, 22 and 24 indicated on the surface of the workpieces, with respect to the time T. It is obvious that in all similar welds a similar form of temperature distribution occurs during the passage of the beam provided that the workpieces are of the same dimensions and extend sufficiently in the direction of the weld seam and that the operation is carried out with similar weld parameters. Therefore, if the temperature is recorded at two equidistant points to the right and left of the weld scam in the section A,,B,, which moves progressively with the electron beam through the workpieces, then if the beam impinges the contact surface 6 of the workpieces exactly symmetrically then the same temperature is measured at equal distances on both sides. If the beam diverges from the seam line, then there is immediately a change in the relative energy applied to both workpieces, this leading to a considerable temperature difference between two workpieces.

To illustrate this situation described, the temperature distribution occurring during electron beam welding to the right and left of the weld seam on the workpiece surface is shown in the graph of FIG. 5 for a stationary section at the times t t, and t,, where t, corresponds substantially to the weld time and t t are later times. X is the lateral clearance from the weld seam 6, and T the temperature. It is clear that the lower flanks of t are so steep that slight displacements occurring in the X-direction are indicated by considerable temperature changes.

A special situation arises during electron beam welding in that this process is generally carried out in a vacuum or highly reduced air pressure (e.g., inert gas pressure). As shown by experience, the heat transfer between two workpiece surfaces in close contact with one another in a vacuum is very bad even with perfect conditions, since the thin gas layer normally located between the two surfaces at normal pressure is missing, and this layer plays a substantial part by heat conduction in the energy transfer. Even under best conditions the workpiece surfaces make contact with one another only at a few points if no special measures are taken. With this bad heat contact an asymmetric energy feed at the weld point causes considerable changes in the temperature distribution. This is particularly noticeable when the electron beam misses the seam in places and feeds energy only to one side.

In accordance with the invention the phenomena described are used to recognize faulty welding operations. Examples of such cases are shown in FIGS. 6 to 8, and 9 to 11 when welding two workpieces 2, 4 along a seam 6 by means of an electron beam 8, which is progressively moved in the direction of arrow 26 along the seam or butt joint 6 and which leaves a melt region 28 behind. Lines l0, l2 and 14 are further isotherms. FIGS. 6 to 8 show the case of the lower end of the electron beam diverging from the course of the seam. The temperature distribution graph occurring here has a typical configuration. In FIGS. 9 to 11 the case is similar. Herein it is assumed that the electron beam is curved by a magnetic field located in the region of the workpieces and hence misses the central portion of the seam, whilst at the top and bottom it is symmetrically located and the weld created has an externally satisfactory appearance. For this case too a typical form of the isothenns results, from which the kind of fault and its position may be ascertained.

Generally, faults as shown in FIGS. 6 to 11 do not occur evenly over the whole length of the weld seam. FIGS. 12 to 14 show a case in which the fault shown in FIGS. 9 to 11 occurs only over a short section of the weld seam. A clear indentation in the course of the isotherms is obtained.

Besides the examples described herein, which concern divergences of the energy fed from the position of the connecting surfaces, there are also other faults which may be caused by an unwanted temperature cycle itself. This involves, e.g., cavity and pore formation, undesired precipitations or metallurgical changes, tendency to microcrack formation and so on. It has been shown that such faults can be substantially eliminated when the energy feed is controllable with sufficient flexibility and reproducible within sufficiently close limits. This is especially the case in electron beam welding, where the kind of energy feed into the weld position by optical beam means is versatile, controllable and predeterminable by optical means. In this manner the temperature cycle may be readily controlled in accordance with welding technique. There is however a technical difficulty in that the electron beam must be controlled by the weld with sufi'rcient reliability as to secure the required thermal distribution in the workpieces. This enables the process of the invention to be used so that the temperature distribution is measured at suitable points in or on the workpieces or along it in similar positions relative to the weld point, and these measuring values are employed directly to control the electron beam or for indicating the condition of the weld.

FIGS. 15 to 18 refer to thermal weldings of two workpieces 2, 4 along a butt surface or seam 6, and show some possible arrangements of measuring elements when the measuring elements together with the thermal welding tool, e.g., an electron beam, are moved relative to the workpieces. The action point of the tool is denoted by 8,'and the arrow 26 indicates the direction in which the tool is moved relative to the workpieces to form the weld seam 28.

FIG. 15 shows simple case which gives satisfactory results for most cases. On both sides of the seam 6 close to the acting point 8 of the thermal tool there are measuring elements 32, 34 located symmetrically with respect to the seam. Depending upon the welding speed used it may also be preferable to locate the measuring elements somewhat in front or somewhat behind the action point 8. The arrangement shown in FIG. 15 enables asymmetries in the energy supply, as produced by an electron beam not accurately set on the seam 6, to be readily recognized as 'a difference between the temperatures measured by the elements 32, 34, and the sign of the difference indicates the direction of deviation from the ideal symmetrical 'tion that the workpieces are energy supply. Inequalities in the workpiece material are also detected.

FIG. 16 shows an arrangement in which in addition to the measuring elements 32, 34 already described a further two pairs 36, 38 and 40, 42 of measuring elements are provided. Thus, the pair 36, 38 located furthermost forward will most clearly reproduce the conditions during the energy supply and during the melting operation, while the measurements from the pairs 32, 34 and 40, 42 located furthest behind are influenced to a greater or lesser extent by the cooling processes in the melting region 28. Thus, a differentiated weld picture is obtained.

FIG. 17 shows an arrangement in which besides the pair 32, 34 shown in FIG. 15 a pair 44, 46 located further forward and close to the seam 6 is used.

FIG. 18 shows additional pairs 48, 50 and 52, 54 which are at a greater distance than those in FIG. 17 from the seam 6. Such measuring elementslocated further outwards are particularly useful in supplying a full picture of thermal distribution.

In most cases when producing joints which extend between two sides of a workpiece, measuring elements are provided on both sides to increase the reliability of measurement and to allow undesired divergences from the normal of the tool relative to the work to be recognized.

FIG. 19 shows a method of application of measuring elements to the workpieces. Although in some cases, such as when producing high-quality individual welds or a small series thereof it would be advisable to mount the measuring elements firmly on the workpieces, as by cementing or welding, it may be more expedient in most cases to provide a detachable connection between the measuring elements and the workpieces. In FIG. 19 a holder 56 is used in the form of a plate 58 which is applied to the surfaces of the workpieces 2, 4, the plate 60 having lugs for locating it on the workpieces; the plate 58 has a slot 62 to allow the thermal tool to pass through to the workpieces 2, 4, and it carries measuring elements 64, 66 such that when the plate 58 is lowered on to the workpieces 2, 4, the measuring elements are firmly held against the surfaces of the workpieces. It is possible either to observe constantly the signals of all measuring elements, and compare them with optimum patterns ascertained by tests, or intermittently, using only the measuring elements located at a predetermined distance from the point of application of the tool. In this manner progressive movement of the measuring positions is obtained in agreement with the movement between the tool and the workpieces.

FIG. 20 shows a case in which the measuring elements together with the tool are moved relative to the workpieces 2, 4. For this purpose the measuring elements 68, 70 are mounted on a holder 72 which is stationary relative to the tool, and thus displaceable relative to the workpieces 2, 4 so that the said elements rest against the surfaces of the workpieces and during the relative movement denoted by the arrow 74 between the workpieces and the tool, move along the workpieces. The holder 72 has a slot 76 for the passage of the tool.

If measuring elements are used which operate as radiation receivers then as narrow a spacing as possible is left between the measuring elements and the workpieces; this method of operation, however, has the disadvantage that the measurements are highly dependent upon the nature of the workpiece surfaces.

The examples shown herein are so far based on the assumpcylindrical in the direction of welding. The general case, includes any normal workpiece shape. The distribution or arrangement of the measuring points are accordingly adapted to the varying shapes of the workpieces. Thus, the measuring means can be applied not only to all external accessible points of the workpieces, but also to any internal surfaces. This includes both cavities enclosed by material and also measuring points made in the material itself.

All heat supply and discharge operations acting on the workpiece, regardless as to whether caused by heat conduction or heat supply and irradiation or by other contemporaneous processes, influence the observed temperature distribution. Thus, holding and clamping devices can cause temperature deviations. Likewise it is possible for the spacing and temperature of the walls and articles surrounding the workpiece to affect the workpiece temperature. In accordance with the invention all these influences are taken into account so that optimum weld conditions are obtained during seam production.

FIG. 21 shows an arrangement capable of operating various forms of the process concerned. Two workpieces 2, 4 which are held together in a holding device, not shown, are welded together along a contact surface or seam 6 by means of an electron beam 78. The beam 78 is produced from a source 80. Since electron beam welding is known as such, conventional devices such as focusing devices, vacuum pumps and the like are not described herein, but may be assumed to be present.

The workpieces rest on two rods 82, 84, which are longitudinally displaceable in grooves in two supporting rails 86, 88.

The rails 86, 88 are rigidly connected with sidewalls at their ends and together therewith form a cradle which is mounted on a base frame pivotable about an axis 90. The figure only shows one sidewall 92 and an associated part of the base frame 94. The pivotal axis 90 is coincident with the upper edge of the seam 6. A driving motor M1 is geared to move the rods 82, 84, the motor being mounted on the sidewall 92 and carrying a pinion 96 which engages a rack 98 on the rod 82. To pivot the cradle a driving motor M2 is used which is mounted on the base frame 94 and operates on a racked arch 100 connected with the side wall 92, concentric with the pivotal axis 90.

A further driving motor M3 is provided which is adapted to displace the base frame 94 in the direction of the double arrow 102. A gear 104 which is mounted ori a control shaft 114 fitted with a plurality of cam discs 106, I08, 110, 112 meshes with the teeth 98 on the rod 82. The cam discs cooperate with feelers and produce signals in signal generators 116, 118, 122 which are formed as Potentiometers; these signals are supplied 'to amplifiers V11, V12, V13, V14.

The temperature distribution in the workpieces 2, 4 is sensed by 20 measuring elements, of which five (Ell-E15) are located on the upper surface of one workpiece 2, five (E22-E25) on the underside of one workpiece 2, five (EM-E35) on the upper surface of the other workpiece 4 and five (EAL-E45) on the underside of the other workpiece 4, symmetrically about the connecting surface 6. The measuring elements are stationary relative to the workpieces 2, 4 and can be mounted as previously described on a separate holder (not shown in FIG. 21), which is detachably connected at a predetermined place to the workpieces.

The measuring elements are connected in the manner shown to amplifiers V1, V2, V3 and V4 such that the outputs 124, 126, 128, of these amplifiers show the sum of the temperature signals supplied to the connected measuring elements. This is obtainable from measuring elements including thermoelements by series connection of the thermoelements. In the arrangement shown in FIG. 21, a signal appears in the output of the amplifier V] which corresponds to the sum of the temperature measuring signals of the measuring elements El 1 to E14.

The outputs 124 and 126 of the amplifiers V1 or V2 are connected to the inputs of a further amplifier V5, which establishes the difference between the output signals from V1 and V2. A further amplifier V6 establishes the difference between the output signals from V3 and V4, an amplifier V7 shows the difference between V1 and V3, and an amplifier V8 shows the difference between V2 and V4. Depending upon the kind of control or indication required, a greater or lesser number of amplifiers described will suffice. For example, for thin workpieces, measuring elements may be provided on one 751 side only.

It can readily be seen that with a symmetrical arrangement of identical measuring elements on workpieces which in a sufficiently large region around the seam 6 can be regarded as identical, the output signals from V1 and'V3 and the output signals from V2 and V4 are substantially identical with one another so long as welding is effected throughout with an even distribution of the welding energy between the workpieces. The amplifiers V7 (=V1V3) and V8 (=V2-V4) then supply a zero output signal, and the amplifiers V5 (=V1-V2) and V6 (=V3-V4) supply substantially equal output signals but varying upwards from zero (since the beam energy on the underside of the workpiece is lower than on the upper surface). If deviations from these simple relationships occur welding faults may be assumed. The signals may be used to control or regtlate the welding operation. Thus, the occurrence of an output signal at V7 (=V1-V3) indicates that the beam 78 does not accurately impinge on the connecting surface, and it is then possible with the aid of the driving motor M3 to displace the workpieces relative to the beam, until V7 again shows a zero output signal. This process can be automated; in FIG. 21 a suitable regulating device 132 is indicated, the function of which requires no detailed explanation.

When V8 (=V2-V4) supplies a relatively large output signal, but V7 remains close to zero, the beam 78, though still correctly entering the connecting surface 6, deviates to one side in the deeper workpiece regions. In this case compensation may be provided by tilting the workpiece about the axis 90 by means of the driving motor M2. This operation may also be automated, the regulating operation being controlled by a signal combination from V7 and V8. In FIG. 21 the regulating device 134 controlling the driving motor M2 is controlled only by V8 (=V2-V4). Normally the correction by tilting or pivoting the work may be dispensed with, so that the parts provided therefor can be omitted (M2, V8, 100, 134). The output signals of the amplifiers V5 (=V1-V2) and V6 (=V3-V4) may be regarded as a measure of the uniformity of the weld seam in the direction of its depth. Accordingly these signals may be used to control or regulate the beam intensity. Suitable devices therefor are available but are not shown in FIG. 21.

When the beam follows a symmetrical welding path the output signals of the amplifiers V5 (=V l-V2) and V6 (=V3-V4) are substantially identical with one another, so that the difference VS-V6 can be used for controlling and regulating purposes. 7

Finally, a signal may also be obtained which is derived from the welding speed; for example, from the rate of change of the output signal from V1 or V3, or from the magnitude of the means value of the output signal from V1 or V3. The faster the relative movement between beam and workpieces the greater the rate of change in the outputs of the summarizing amplifiers V1 and V2, and the smaller the mean value of the signals in these outputs.

In many cases a joint to be produced is asymmetrically located in the workpieces, so that the above described criteria can only be applied after greater or lesser adaptations. Nevertheless even in such cases a very reliable monitoring is possible. For this purpose a number of welding tests are previously carried out with similar workpieces under varying conditions. Thus, differing weld parameters (beam geometry, beam output or welding speed) are used.

The welded joints obtained are destructively tested and the results show which of the tests carried out provides best results. In all tests the temperature distribution occurring during welding is recorded.

Thereafter workpieces with the same weld parameters as the test pieces are welded under the conditions which brought the best results, again referring to the temperature distribution obtained. If the latter agrees with the temperature distribution of the optimum test sample, then it is certain that the weld is the best possible. This reliability can be increased by increasing the number of tests and using numerous measuring points so that the result conforms to the degree of reliability derivable from the mechanical characteristics of the material or the semifinished articles produced.

FIG. 21 shows how the described comparison of the temperature distribution during welding can be conveniently carried out so as to correspond with the temperature distribution ascertained during tests. For this purpose a program generator with suitably shaped cam discs, 106, 108 and 112 is provided. The cam disc 106 via its feeler and the associated signal generator V11 supplies a signal form during the progressive feed of the workpiece relative to the electron beam used for welding, which form corresponds to the optimum signal form ascertained from the preliminary tests at the output of the amplifier V1. Similarly the can disc 108 supplies an optimum signal form with regard to the amplifier V12, the cam disc 110 for V3 and the cam disc 112 for V4.

From the signaldifferences Vll-Vl, V12-V2, V13-V3 and V14-V4 which are obtained from a comparison device (not shown), a measurement is obtained of the digression of the weld from the optimum path. Thus the signals supplied by the cam discs 106, 108, 110 and 112 are nominal values, the signals supplied by the amplifiers V1, V2, V3 and V4 are actual values, and the differences between nominal values and actual values form correction signals. These signals can be used either forregulating and control purposes, or to select workpieces in which the deviations from the optimum temperature distribution exceed predetermined limits.

The above statements make it clear that not all the means shown in FIG. 21 are necessary, depending upon the conditions prevailing in'the practical application of the invention. More especially when using up-to-date welding apparatus such as that using electron beam welding, the reproducibility of the beam current and beam control is so good that reregulation of once set weld parameters can be dispensed with in many cases, so that actual process control, by means of program generators 116-122, V11V14 is readily possible.

If, as described in connection with FIG. 21, it is desired to have both regulating operations and process or material control, the problem arises as to how to ascertain whether measured deviations or changes of temperature distribution originate from welding faults or material faults, or from regulating errors which are correctable. Such differentiation, however, is readily possible since the changes of temperature distribution caused by welding faults or material faults nearly always occur suddenly as compared with normal regulating deviations. This difference may be utilized by employing circuits which have delay and differentiation members to separate the control signals from the observed signals.

It is obviousthat with greater demands for accuracy of the temperature distribution observations a correspondingly greater number of measuring elements must be used in an appropriate distribution, e.g., two or three rows of measuring elements on both sides of a seam.

When using measuring elements which are stationary relative to the workpieces in the manner shown in FIG. 21, it is possible to use only the measuring elements close to the tool, which can be effected by a multipoint switching device (not shown), depending upon the relative movement between tool and workpieces.

It should be understood, more particularly in connection with long connecting seams, that measuring elements are used which are moved with the tool relative to the workpieces (see e.g., FIG. 20). In this case, the statements made in connection with FIG. 21 apply accordingly and the measuring elements El 1-E15, E2l-E25, 1531-1335 and 1541-1345 may be imagined as stationary relative to the beam source 80.

FIG. 22 shows an arrangement in which the temperature distribution has no effect on the workpieces themselves, but produce an image of the workpiece. The workpieces 2, 4 which are to be welded together along a seam 6 by means of an electron beam 78, are arranged in the working chamber 136 of the electron beam welding device, which is evacuated. An infrared image system 138 projects an image 2', 4' of the workpieces onto an image screen 142 via a feeler and control circuit 140, together with their seam 6' and the weld region 28'. The brightness of the individual image points is made to correspond to the temperature of the associated workpiece sonic energy. A

point. On the image screen 142 a holder 146 is displaceable by means of a motor 148 in the direction of the image 6' of the weld seam. The motor 148 is driven synchronously with the feed of the workpieces 2, 4 in the working chamber 136, so that the measuring elements 148-158 mounted on the holder 146 which may be photoelectric cells or photoelements responding to visible light, together with the image 3 of the beam incident area, move along the image 6' of the seam.

Arrangements of the kind shown in FIG. 21 have the advantage that changes in the distribution of measuring elements The test procedure proceeds in exactly the same way as when I producing a mechanical joint or weld.

FIGS. 23 to 27 show embodiments of the invention in which other parameters of the material than temperature are used.

FIG. 23 shows a view similar to FIG. 1 Le, a plan view of two workpieces 2 and 4, which are to be butt welded to one another by means of an electron beam at the contact surface 6. The electron beam entering at right angles to the plane of the drawing at the contact surface 6 is moved relative to the workpieces 2 and 4 in the direction of arrow 26 and leaves a joint 28 behind.

Close to the outer surface of the workpiece 2 there is a supersonic generator 231, whilst at one outer surface of the workpiece 4 there is a measuring instrument 232 in the form of a supersonic receiver. The contact surface 6 of the workpieces before the weld constitutes a reflection surface, since the workpiece surfaces owing to their irregularities and roughness are only in contact in a few places. In these circumstances the measuring instrument 232 receives only little or no supersonic energy directly through the contact surface 6 not yet welded. In contrast thereto, in the region where the welding electron beam has already produced a molten connection or momentary weld region between the two workpieces 2 and 4 which is indicated in FIG. 23 at 233, a perfect transmission of supersonic energy takes place, so that the measuring element 232 receives a high proportion of supersonic energy via this region. The connecting region 28 located behind the momentary weld region and already set, if it is faultless, allows a substantially uniform and reflection-free passage of supercontinuously rising supersonic signal is thus received by the measuring element 232 during the progress of the welding operation. Hence it is seen that a similar control is possible to that of the embodiments described in the previous examples, in which the workpiece temperature is used as the observed parameter.

The process shown in FIG. 23 operates by supersonic impulses. The transit time of an impulse between the generator 231 and the measuring element 232 is measured. Since the shortest possible impulse path extends over the momentary weld seam 233, the transit time measured will become smaller as the weld progresses. From the kind of signals occurring at the measuring element 232 conclusions may be drawn as to the quality of the weld established.

A particularly sensitive variation of the process of FIG. 23 is obtained if an interference measurement is carried out. This is shown in FIG. 24 which is similar to FIG. 23. On an outer surface of the workpiece 2 two supersonic generators 231 and 234 are mounted spaced apart, while on the outer surface of the workpiece 4 there are two corresponding measuring elements or receivers 232 and 235. The generators 231 and 234 are operated in synchronism and each of the receivers 232 and 235 receives intensities which correspond to the interference pattern changes with the progress of the weld, since for the same conditions of permeability of the sonic energy through the connecting region apply as in FIG. 23.

The two measuring elements or receivers 232 and 235 shown in FIG. 24 may also be used to effect a reflection measurement which corresponds to the interference pattern of the two sonic sources 231 and 234; the result may be used as a measure of the quality of the weld obtained. FIGS. 25 to 27 show the use of a magnetic alternating field producing circulating currents as means of evaluating the progress of a weld. This process is suitable for electrically conductive workpieces.

FIG. 25 in turn shows a plan view of two workpieces 2 and 4 which are to be joined along a contact surface 6. A welding electron beam 8 is indicated in cross section in the figure as a thermal tool for producing the weld. By means of a signal generator 260, which produces a magnetic alternating field, circulating currents are induced in the workpiece material in the weld region, and these currents in turn produce magnetic fields, so that the measured magnetic flux density in the joint region of induction B depends upon the intensity and spatial distribution of the said currents. Hence, by measuring the density B of the magnetic flux an estimate of the quality of the weld obtained may be received during the welding process. For this purpose measuring elements 262, 264, 266 and 268 are indicated in FIG. 25 which respond to changes in the magnetic flux. The signal generator 260 schematically indicated in FIG. 25 is preferably moved together with the electron beam 8 relative to the workpieces 2 and 4. The measuring elements 262 to 268 may be stationary relative to the workpieces; similar conditions are then obtained as in stationary thermal measuring elements included in embodiments described above. It is, however, also possible for the measuring elements 262 to 268 to be moved together with the beam in the direction of the arrow 26 relative to the workpieces 2 and 4; in this case thepair of measuring elements 262, 266 located be- [hind the beam in the connecting region 28 already cooled and set, will constantly supply a typically different measurement from the pair of measuring elements 264, 268 located in front of the beam.

Particularly instructive information is obtained if at least one measuring element is additionally moved, transversely to the direction 26, so as to sweep over a relatively large region and consequently supply successive information concerning various parts of the magnetic field existing at or close to the weld point (in the incidence region of the electron beam 8). The signals so obtained may be used to produce an image of the magnetic field distribution on the image screen of an oscillograph. Such an image immediately shows faults occurring during the welding process, since they cause an asymmetrical field distribution.

The movement of at least one measuring element is often possible also in connection with measuring elements which sense other state values, such as a sonic pressure.

FIGS. 26 and 27 show typical distributions of the magnetic alternating flux density B in the direction x at right angles to the connecting seam 6, 28. FIG. 26 corresponds to the state in the region of the completed weld region 28. FIG. 27 corresponds to the state in the region of the unwelded contact surfaces 6. Of course, the actual distribution of the magnetic alternating field flux density B also depends upon the type and arrangement of the signal generator 26.

Besides the embodiments described, further embodiments are possible by using other state values. In any case it is possible by using a method in accordance with the invention, to obtain reliable data concerning the quality of a joint already made, during the welding operation.

I claim:

1. A method of monitoring the state of mechanical joints produced by joining two workpieces by a thermal joining process such as welding which comprises sensing, at the workpieces, during the joining process, values existing in the workpieces and dependent on changes of state occurring in the pattern of both sound sources 231 and 234. This interference area where the joining takes place, said sensing taking place at predetermined sensing locations remote from said joining area, and utilizing said sensed values as a measure of the state of the joint produced.

2. A method as recited in claim 1, in which said values are sensed at a plurality of predetermined sensing locations which are spaced from each other and are remote from said joining area.

3. A method as claimed in claim 2 as applied to the production of a joint along a seam, including the steps of moving a thermal tool relative to said workpiece along said seam, and sensing the values of a particular quantity of state at sensing locations arranged on both sides of said seam and spaced therefrom.

4. A method as recited in claim 3, including the step of moving said sensing locations together with said tool.

5. A method as recited in claim 3, including the step of fixedly locating said sensing locations with respect to said workpiece.

6. A method as recited in claim 4, comprising the step of effecting movement of said sensing locations by successively taking sensing signals from different sensing locations which are stationary with respect to said workpiece.

7. A method as recited in claim 1, comprising the steps of producing an image of the area where said joint is to be made, said image corresponding with the distribution of at least one sensed variable quantity of state in said workpiece, said distribution being represented by the intensity distribution of said image.

8. A method as recited in claim 7, including the step of transposing said sensed variable quantity of state into visible light, the distribution of said quantity being represented by the light intensity distribution of said image.

9. A method as recited in claim 1, applied to repeated production of like joints in like workpieces, including the steps of performing a number of preliminary tests to determine an optimum distribution of a quantity of state corresponding to optimum processing by applying predetermined different working conditions, sensing and recording the associated distribution as to time and space of said quantity of state, examining to destruction the joints obtained; thereafter applying, in the actual production of said joints, those working conditions which correspond to said optimum processing, sensing the distribution of said quantity of state occurring during said actual production, and discarding any joint obtained in which the deviation between the sensed and optimum distributions of said quantity of state is outside a predetermined tolerance range.

10. A method as recited in claim 9, including selecting the number of preliminary tests in accordance with a desired degree of security of the joint.

11. A method as recited in claim 1, including deriving a signal from at least one distribution of a sensed quantity of state, and utilizing said signal for controlling an operation of said thermal jointing process.

12. A method as recited in claim 3, including sensing the magnitudes of quantity of state at sensing locations which are disposed in front of said tool in the direction of the movement of said tool.

13. A method as recited in claim 1, comprising utilizing the temperature at predetermined parts of said workpiece as the quantity of state which reflects the changes of the state of the joint produced.

14. A method as recited in claim 1, comprising the step of feeding input signals into at least one input location of said workpiece to be connected during said jointing process, said input signals being of a kind capable of propagation within the workpiece material, and sensing a quantity of state which is dependent on said input signals and on the jointing condition prevailing in said jointing area.

15. A method as recited in claim 14, wherein said input signals are pressure signals, and wherein said quantity of state is a pressure signal which is sensed at at least one sensing location of said workpiece, said sensing location being spaced from said input location.

16. A method as recited in claim 15, wherein said pressure signals are ultrasonic signals.

17. A method as recited in claim 16, including the step of measuring the propagation time of said signals between said input location and said sensing location.

18. A method as recited in claim 16, wherein a reflection of said signals is sensed in the area of said joint being produced.

19. A method as recited in claim 16, comprising the steps of synchronously feeding sonic signals into at least two input locations, and sensing the resultant acoustic interference pattern at at least one location.

20. A method as recited in claim 14, utilizing magnetic signals as said input signals, and sensing a magnetic signal as said quantity of state at at least one sensing location.

21. A method as recited in claim 20, comprising the steps of utilizing an alternating magnetic field as said input signal, and sensing a magnetic flux change to evaluate said quantity of state.

22. A method as recited in claim 21, including sensing said magnetic flux density, and supplying the alternating magnetic field to the area of the joint being produced.

23. A method as recited in claim 14, applied to the production of a joint between two workpieces along a seam, including the steps of moving an electron beam thermal tool along said seam relative to said workpieces, and moving said signal input location along together with said tool.

24. A device for making a thermal joint between a pair of meltable workpiece materials, comprising a source of heat, means for producing relative movement between said workpieces and said heat source so that heat is progressively applied along the line of the joint to be made, a plurality of sensing elements arranged on said workpieces in a predetermined pattern relative to said line, said sensing elements being remote from said line and being responsive to values existing in the workpieces and dependent on changes of state occurring during the joining process in the area where the joining takes place, and means for evaluating signals received from said sensing elements.

25. A device as recited in claim 24, comprising sensing elements provided on both sides of said workpiece if a joint extends partly or totally between the two sides of said workpiece.

26. A device as recited in claim 24,,wherein, in the production of a joint along a seam, sensing elements are provided on both sides of said seam and remote therefrom.

27. A device as recited in claim 24, comprising a support, sensing elements arranged in a predetermined pattern on said support, and means for connecting said support with said workpieces, the arrangement being such that the sensing elements become effective at predetermined sensing locations on the workpieces upon interconnection of said support with said workpieces.

28. A device as recited in claim 24, comprising a support, said sensing elements being carried on said support, means for moving said support relative to said workpieces together with said thermal too], said sensing elements becoming effective as measuring elements at; predetermined surface areas along predetermined paths of said movement.

29. A device as recited in claim 28, wherein said means for moving said support provides yielding engagement between said workpieces and said sensing elements.

30. A device as recited in claim 24, comprising a programmer adapted to supply, synchronously with the progress of said jointing action, a program of desired values with respect to the signals obtained from said sensing elements during the jointing action.

31. A device as recited in claim 30, comprising a comparator adapted to compare the program values with the sensed signals, and to produce corresponding differential signals therefrom.

32. A device as recited in claim 24, comprising control means controlled by said signals of said sensing elements, and adapted to modify parameters of said jointing process in such a manner that said sensed signals remain within a predetermined range of values.

33. A device as recited in claim 24, comprising an imaging system, means causing said imaging system to respond to temperature differentials produced during the joining process in the region of the workpieces to be joined, said sensing elements being arranged on the image surface of said imaging system and separated from said workpieces.

34. A device as recited in claim 24, wherein said sensing elements are temperature sensors.

35. A device as recited in claim 24, wherein said sensing elernents respond to the magnetic permeability of said workpieces.

36. A device as recited in claim 24, comprising a signal transmitter coupled to at least one predetermined input location of a workpiece to be joined, said signed transmitter supplying a signal which is capable of being propagated within said workpiece and responsive to said changes of state and which is receivable by said sensing elements.

37. A device as recited in claim 36, comprising means for moving said signal transmitter relative to said workpiece together with said thermal tool producing said joint.

38. A device as recited in claim 36, comprising means for coupling said signal transmitter to one of said workpieces, wherein at least one sensing element is arranged on the other of said workpieces so that the proportion of the signal supplied by said signal transmitter which is received by said sensing element, is dependent on the condition of said joint.

39. A device as recited in claim 36, wherein said signal transmitter is an alternating pressure transmitter.

40. A device as recited in claim 38, comprising two synchronously operated alternating pressure signal transmitters coupled to said one workpiece at a predetennined distance apart in the direction of said joint.

41. A device as recited in claim 40, wherein two sensing elements are located on the other of said workpieces at a predetermined distance apart.

42. A device as recited in claim 24, for the production of joints between electrically conductive workpieces, comprising at least one signal generator producing an alternating magnetic field, and at least one sensing element capable of responding to alternating magnetic flux, the density of which responds to said changes of state, said generator being located to produce said field in an area including the location where said thermal tool produces said joint.

43. A device as recited in claim 42, comprising means for moving at least one sensing element so that said sensing element senses during its movement different portions of said magnetic field. 

1. A method of monitoring the state of mechanical joints produced by joining two workpieces by a thermal joining process such as welding which comprises sensing, at the workpieces, during the joining process, values existing in the workpieces and dependent on changes of state occurrIng in the area where the joining takes place, said sensing taking place at predetermined sensing locations remote from said joining area, and utilizing said sensed values as a measure of the state of the joint produced.
 2. A method as recited in claim 1, in which said values are sensed at a plurality of predetermined sensing locations which are spaced from each other and are remote from said joining area.
 3. A method as claimed in claim 2 as applied to the production of a joint along a seam, including the steps of moving a thermal tool relative to said workpiece along said seam, and sensing the values of a particular quantity of state at sensing locations arranged on both sides of said seam and spaced therefrom.
 4. A method as recited in claim 3, including the step of moving said sensing locations together with said tool.
 5. A method as recited in claim 3, including the step of fixedly locating said sensing locations with respect to said workpiece.
 6. A method as recited in claim 4, comprising the step of effecting movement of said sensing locations by successively taking sensing signals from different sensing locations which are stationary with respect to said workpiece.
 7. A method as recited in claim 1, comprising the steps of producing an image of the area where said joint is to be made, said image corresponding with the distribution of at least one sensed variable quantity of state in said workpiece, said distribution being represented by the intensity distribution of said image.
 8. A method as recited in claim 7, including the step of transposing said sensed variable quantity of state into visible light, the distribution of said quantity being represented by the light intensity distribution of said image.
 9. A method as recited in claim 1, applied to repeated production of like joints in like workpieces, including the steps of performing a number of preliminary tests to determine an optimum distribution of a quantity of state corresponding to optimum processing by applying predetermined different working conditions, sensing and recording the associated distribution as to time and space of said quantity of state, examining to destruction the joints obtained; thereafter applying, in the actual production of said joints, those working conditions which correspond to said optimum processing, sensing the distribution of said quantity of state occurring during said actual production, and discarding any joint obtained in which the deviation between the sensed and optimum distributions of said quantity of state is outside a predetermined tolerance range.
 10. A method as recited in claim 9, including selecting the number of preliminary tests in accordance with a desired degree of security of the joint.
 11. A method as recited in claim 1, including deriving a signal from at least one distribution of a sensed quantity of state, and utilizing said signal for controlling an operation of said thermal jointing process.
 12. A method as recited in claim 3, including sensing the magnitudes of quantity of state at sensing locations which are disposed in front of said tool in the direction of the movement of said tool.
 13. A method as recited in claim 1, comprising utilizing the temperature at predetermined parts of said workpiece as the quantity of state which reflects the changes of the state of the joint produced.
 14. A method as recited in claim 1, comprising the step of feeding input signals into at least one input location of said workpiece to be connected during said jointing process, said input signals being of a kind capable of propagation within the workpiece material, and sensing a quantity of state which is dependent on said input signals and on the jointing condition prevailing in said jointing area.
 15. A method as recited in claim 14, wherein said input signals are pressure signals, and wherein said quantity of state is a pressure signal which is sensed at at least one sensing location of said workpiece, said sensing location being spaced from said input location.
 16. A method as recited in claim 15, wherein said pressure signals are ultrasonic signals.
 17. A method as recited in claim 16, including the step of measuring the propagation time of said signals between said input location and said sensing location.
 18. A method as recited in claim 16, wherein a reflection of said signals is sensed in the area of said joint being produced.
 19. A method as recited in claim 16, comprising the steps of synchronously feeding sonic signals into at least two input locations, and sensing the resultant acoustic interference pattern at at least one location.
 20. A method as recited in claim 14, utilizing magnetic signals as said input signals, and sensing a magnetic signal as said quantity of state at at least one sensing location.
 21. A method as recited in claim 20, comprising the steps of utilizing an alternating magnetic field as said input signal, and sensing a magnetic flux change to evaluate said quantity of state.
 22. A method as recited in claim 21, including sensing said magnetic flux density, and supplying the alternating magnetic field to the area of the joint being produced.
 23. A method as recited in claim 14, applied to the production of a joint between two workpieces along a seam, including the steps of moving an electron beam thermal tool along said seam relative to said workpieces, and moving said signal input location along together with said tool.
 24. A device for making a thermal joint between a pair of meltable workpiece materials, comprising a source of heat, means for producing relative movement between said workpieces and said heat source so that heat is progressively applied along the line of the joint to be made, a plurality of sensing elements arranged on said workpieces in a predetermined pattern relative to said line, said sensing elements being remote from said line and being responsive to values existing in the workpieces and dependent on changes of state occurring during the joining process in the area where the joining takes place, and means for evaluating signals received from said sensing elements.
 25. A device as recited in claim 24, comprising sensing elements provided on both sides of said workpiece if a joint extends partly or totally between the two sides of said workpiece.
 26. A device as recited in claim 24, wherein in the production of a joint along a seam, sensing elements are provided on both sides of said seam and remote therefrom.
 27. A device as recited in claim 24, comprising a support, sensing elements arranged in a predetermined pattern on said support, and means for connecting said support with said workpieces, the arrangement being such that the sensing elements become effective at predetermined sensing locations on the workpieces upon interconnection of said support with said workpieces.
 28. A device as recited in claim 24, comprising a support, said sensing elements being carried on said support, means for moving said support relative to said workpieces together with said thermal tool, said sensing elements becoming effective as measuring elements at; predetermined surface areas along predetermined paths of said movement.
 29. A device as recited in claim 28, wherein said means for moving said support provides yielding engagement between said workpieces and said sensing elements.
 30. A device as recited in claim 24, comprising a programmer adapted to supply, synchronously with the progress of said jointing action, a program of desired values with respect to the signals obtained from said sensing elements during the jointing action.
 31. A device as recited in claim 30, comprising a comparator adapted to compare the program values with the sensed signals, and to produce corresponding differential signals therefrom.
 32. A device as recited in claim 24, comprising control means controlled by said signals of said sensing elements, and adapted to modify parameters of said jointing process in such a manner that said sEnsed signals remain within a predetermined range of values.
 33. A device as recited in claim 24, comprising an imaging system, means causing said imaging system to respond to temperature differentials produced during the joining process in the region of the workpieces to be joined, said sensing elements being arranged on the image surface of said imaging system and separated from said workpieces.
 34. A device as recited in claim 24, wherein said sensing elements are temperature sensors.
 35. A device as recited in claim 24, wherein said sensing elements respond to the magnetic permeability of said workpieces.
 36. A device as recited in claim 24, comprising a signal transmitter coupled to at least one predetermined input location of a workpiece to be joined, said signed transmitter supplying a signal which is capable of being propagated within said workpiece and responsive to said changes of state and which is receivable by said sensing elements.
 37. A device as recited in claim 36, comprising means for moving said signal transmitter relative to said workpiece together with said thermal tool producing said joint.
 38. A device as recited in claim 36, comprising means for coupling said signal transmitter to one of said workpieces, wherein at least one sensing element is arranged on the other of said workpieces so that the proportion of the signal supplied by said signal transmitter which is received by said sensing element, is dependent on the condition of said joint.
 39. A device as recited in claim 36, wherein said signal transmitter is an alternating pressure transmitter.
 40. A device as recited in claim 38, comprising two synchronously operated alternating pressure signal transmitters coupled to said one workpiece at a predetermined distance apart in the direction of said joint.
 41. A device as recited in claim 40, wherein two sensing elements are located on the other of said workpieces at a predetermined distance apart.
 42. A device as recited in claim 24, for the production of joints between electrically conductive workpieces, comprising at least one signal generator producing an alternating magnetic field, and at least one sensing element capable of responding to alternating magnetic flux, the density of which responds to said changes of state, said generator being located to produce said field in an area including the location where said thermal tool produces said joint.
 43. A device as recited in claim 42, comprising means for moving at least one sensing element so that said sensing element senses during its movement different portions of said magnetic field. 